Hemingray made CD 121 tolls for themselves and for Am Tel & Tel Co. The Am Tels usually have smooth bases, and their Hemingray-embossed tolls usually have drip points. But sometimes they'd mix up the bases. This would result in smooth base tolls embossed with the May 2, 1893 patent date for drip points. Here are all 3 error variants I know of, from left to right:
CD 121 Hemingray  - Early prismatic embossing style
CD 121 Hemingray  - Early prismatic embossing style, missing the "A" crossbar in "MAY"
CD 121 Hemingray  - Later stamped embossing style